1. Field of the Invention
The present invention is related to an optical member. Particularly, the present invention is related to an optical member for use in a surface plasmon resonance measuring apparatus.
2. Description of the Related Art
Plasmon sensors that utilize the principles of the surface plasmon resonance phenomenon using evanescent waves to quantitatively analyze substances within samples are known. U.S. Patent Application Publication No. 20090218499 discloses an apparatus, in which a metal film is provided on a prism. A target substance labeled with a fluorescent substance is caused to bind onto the metal film, and a light beam is caused to enter the interface between the prism and the metal film at an angle that satisfies conditions for plasmon resonance, to generate a strong electric field is generated on the metal film. The fluorescent substance is strongly excited by the electric field, and the fluorescence generated thereby is measured.
It is necessary to cause an excitation light beam to be totally reflected at the interface between a prism and a metal film provided thereon, in order to cause surface plasmon resonance to occur in the vicinity of the metal film that functions as a sensor region. The prism is formed to have a cross sectional shape of an inverted triangle or a trapezoid. A light source emits a converged excitation light beam toward the prism such that the excitation light beam enters the prism through a side surface (also referred to as a light incident surface) and is totally reflected at the metal film. There are cases in which the excitation light beam is repeatedly totally reflected within the prism, and reaches a side surface (also referred to as a light output surface) opposite the light incident surface. In this type of apparatus, the excitation light beam is converged as it enters the prism and caused to be totally reflected at the sensor region at an area substantially the same as that of the metal film. Therefore, the excitation light beam converges then spreads as it propagated toward the light output surface after being totally reflected. In this case, if the excitation light beam is irradiated onto the corner formed by the upper surface of the prism and the light output surface, or onto the corner formed by the lower surface of the prism and the light output surface, components which are regularly reflected at the corner (hereinafter, referred to as returning light) propagate backwards along the optical path and enters the light source of the excitation light beam. If the returning light enters the light source, light emission by the light source is destabilized, and there are cases in which errors occur in the amount of detected fluorescence. In addition, if the excitation light beam is totally reflected at the light output surface of the prism, the excitation light beam is scattered within the prism, and there is a problem that the accuracy of measurement deteriorates. Providing a light shielding plate may be considered as a measure for preventing the returning light from entering the light source. However, because the returning light travels along the same optical path as the excitation light beam, the excitation light beam will also be shielded if the light shielding plate is provided. Adjusting the angle of the excitation light beam is another possible measure for preventing the returning light from entering the light source. However, adjustments to the angle of the excitation light beam are sensitive, and it is difficult for users to perform such adjustments when they utilize the plasmon sensors.